1. Field of the Invention
This invention relates to the elimination or reduction of appearance defects known as "measles", as defined hereinafter, in fluorescent lamps having a starting aid conductive layer or coating on the inner surface of the lamp tube or glass envelope.
2. Background of the Invention
Rapid-start or similar fluorescent lamps including a conductive layer, such as a tin oxide or indium oxide layer, and mercury vapor as part of the discharge sustaining gas fill are subject to the formation of localized appearance defects referred to as "measles." Such defects comprise a dark spot surrounded by a concentric ring of discoloration usually of the order of one or two millimeters in diameter. Measles are believed to develop during lamp operation as a result of an interaction involving the conductive layer and the mercury in arc discharge.
The occurrence of such appearance defects has been delayed in fluorescent lamps having a tin oxide conductive layer by varying the electrical resistance of the conductive layer along the axial length of the glass tube. More particularly, the electrical resistance profile of the conductive layer has been varied from a flat or constant value to a U-shaped or "bathtub" profile wherein a relatively low resistance value exists at the center portion of the lamp and relatively high resistance values exist at the end portions of the lamp. This profile is provided during lamp manufacture by making the tin oxide coating thicker at the ends of the lamp than at the middle of the lamp. This resistance profile in such lamp is a function of physical and chemical characteristics including thickness of the tin oxide layer applied directly to the inner wall surface of the lamp envelope or tube. The relative differences in electrical resistance along the axial length of the lamps achieved in this manner tend to decrease after about the first 500 hours of lamp operation. Moreover, the resulting variations in electrical resistance merely delay the occurrence of such defects from a time following the first 1000 hours of lamp operation to a later time after about 3000 to 4000 hours of lamp operation. This is a rather short improvement in the total life of the lamp which is of the order of about 20,000 hours. Accordingly, this process technique does not provide a satisfactory solution to such measle defects.